A wireless communication terminal capable of switching two or more communication systems to perform communication with a base station is known.
Known as such a wireless communication terminal capable of performing communication in a plurality of communication systems is, for example, a wireless communication terminal capable of switching between a CDMA2000 1x system intended mainly for voice communication and a CDMA 2000 1xEVDO system used only for data communication, to perform communication using a common antenna (which will be hereinafter referred to as dual system) (for example, refer to patent document 1).
The CDMA 2000 1xEVDO system is a next-generation high-speed wireless communication system, and is standardized in Std.T-64 IS-2000 C.S. 0024 “CDMA2000 High Rate Packet Data Air Interface Specification” in Association of Radio Industries and Businesses ARIB as a system provided by standardizing HDR (High Data Rate) system, provided by U.S.A. QUALCOMM Incorporated, which is an expansion system of the CDMA2000 1x system. The CDMA 2000 1xEVDO system is a system intended for improving the transmission rate by expanding CDMAOne system served by KDDI CORPORATION in Japan at present (ARIBT-53 in Japan, EIA/TIA/IS-95 in North America, Republic of Korea, etc., and the like), and by specializing the CDMA2000 1x system further in data communication, that is made compatible with the third-generation system (3G).
“EV” of CDMA2000 1xEVDO means Evolution and “DO” means Data Only (refer to patent document 2).
In the communication system using such a dual system wireless communication terminal, the CDMA2000 1xEVDO system can vary the upper limit of the transmission rate according to the congestion state of a base station.
The wireless communication terminal in the communication system performs communication based on the upper limit.
For example, for the upper limit value of the transmission rate of communication from the wireless communication terminal to a base station (uplink communication), the lowest speed is selected at the communication start time. After this, the wireless communication terminal changes the transmission rate based on instruction information that instructs to increase or decrease the upper limit value of the transmission rate transmitted from the base station at every predetermined timing.
The base station may transmit urgent information for limiting the upper limit value of the transmission rate to the wireless communication terminal, aside from the information transmitted from the base station at every predetermined timing.
In this case, the wireless communication terminal changes the transmission rate to a limit value based on the received urgent information.
Such urgent information is transmitted in a situation in which it is feared that the base station may become down if the transmission rate is not dropped, such as in a case where a large number of wireless communication terminals are connected and concentrated to one base station up to nearly the limit of the connection permissible amount of the base station (the base station is in a congestion state), for example.
Thus, in the CDMA2000 1xEVDO system, the upper limit value of the transmission rate at least in the uplink communication varies.
Further, in the dual system wireless communication terminal, if the radio wave state between the wireless communication terminal and the base station becomes poor during communication in the CDMA2000 1xEVDO system, the antenna can be switched so as to perform communication by using the CDMA200 1x system which is the other communication system.
Thus, if the wireless communication terminal determines that satisfactory communication cannot be performed by using the CDMA2000 1xEVDO system because the radio wave state becomes poor, the CDMA2000 1xEVDO system can be switched to the CDMA200 1x system to continue the communication.
Switching of communication from the CDMA2000 1xEVDO system to the CDMA200 1x system is called “hand down.”
In a wireless communication system including wireless communication terminals and base stations, such as a cellular communication system, an art is proposed wherein a base station to which a wireless communication terminal connects for performing communication (which will be hereinafter referred to as “connecting base station”), and one or more base stations adjacent to the connecting base station that become candidates of handoff destination (which will be hereinafter referred to as “peripheral base station”) each instruct to increase or decrease the transmission rate based on the congestion degree of the base station, and the wireless communication terminal varies the transmission rate based on the instruction for performing communication control.
In this specification, handoff means switching of the connecting base station.
In the communication control, if all of the connecting base station and the peripheral base stations as candidates of handoff destination is low in the congestion degree and all base stations instruct to increase the transmission rate, the wireless communication terminal operates so as to increase the transmission rate; on the other hand, if one of the base stations is high in the congestion degree and instructs to lower the transmission rate, the wireless communication terminal operates so as to lower the transmission rate.
As the control is thus performed, the transmission rate is matched with that of the congested base station among the connecting base station and all peripheral base stations, so that when the handoff is made from the base station low in the congestion degree and high in the transmission rate to the base station high in the congestion degree and low in the transmission rate, occurrence of an error such as packet loss caused by a rapid decrease in the transmission rate can be prevented.    Patent document 1: Refer to JP-A-2003-298762    Patent document 2: Refer to JP-A-2002-300644